The present invention relates to a method of calibrating an electron beam system for lithography in making patterns on a semiconductor wafer or in making a mask therefor. More specifically, the invention relates to a method of calibrating the electron beam system by using a standard grid and thereby improving the accuracy of XY placement on a stage of the electron beam system.
Electron beam systems, or electron beam tools, are used in lithographic processes called electron beam lithography, or e-beam lithography, to make masks for producing patterns on semiconductor wafers or to produce patterns directly on semiconductor wafers. The electron beam of an electron beam system is electrically and magnetically deflected by a computer-controlled deflection system in a known manner whereby the electron beam writes a desired pattern on a substrate to produce the mask or writes a desired pattern directly on a semiconductor wafer. All electron beam systems have inherent errors. As a result, distortions are present in the pattern written by the electron beam unless the electron beam system is calibrated to correct for such errors.
A known method of calibrating an electron beam system uses a two-dimensional standard grid and an algorithm for adjusting the computer-controlled deflection system for deviations of the electron beam from the standard grid due to the errors inherent in the electron beam system. This known method is a self-calibration method, which involves the use of an imperfectly constructed measurement gauge, or standard, and an imperfectly calibrated measuring or manufacturing machine to calibrate each other. A number of algorithms exist for self-calibration of electron beam systems through the use of metrology standards, such as standard grids, in two dimensions. Such algorithms are described in, for example, Statistical Perspectives of Self-Calibration, Raugh, M. R. et al., Proceedings of SPIE, Vol. 2725, pp. 114-121, April 1996; Error Estimation for Lattice Methods of Stage Self-Calibration, Raugh, M. R., Proceedings of SPIE, Vol. 3050, pp. 614-625, 1997; Overlay Can Be Improved by Self-Calibrated XY Measuring Instrument: A Lattice Perspective, Raugh, M. R., Proceedings of SPIE, Vol. 2884, pp. 379-391, July 1996; and Obtaining a Physical Two-Dimensional Cartesian Reference, Takac, M. T. et al., Journal of Vacuum Science Technology, B 15(6), pp. 2173-2176, November/December 1997.
All metrology or calibration standards have their own built-in errors. These errors must be characterized so that their effects can be compensated in calibration methods. Typically, a two-dimensional grid is physically rotated so that errors due to its orthogonality and symmetry can be nulled. Thus, one requirement of all of the methods using metrology standards is that the standard to be calibrated, such as a standard grid, must be able to undergo both translation and rotation during the calibration process. However, standard grids for an electron beam system must be fixed inside the system, and the freedom to both translate and rotate the standard grid is not usually available due to, for example, mounting requirements. Furthermore, even when the freedom is available, a very precise mechanical function is required.